Various forms of encoding have been used in the transmission of binary digital signals. The efficiency of such systems can be increased by using block codes of which the 4B-3T, MS43, and FOMOT block ternary codes are well-known examples. A comparison between these three codes is discussed in a paper entitled "Ternary Line Codes" by J. B. Buchner, Philips Telecommunication Review, Vol. 34, No. 2, June 1976, pages 72-86. The MS43 code is also described in U.S. Pat. No. 3,587,088 entitled "Multilevel Pulse Transmission Systems Employing Codes Having Three or More Alphabets" issued June 22, 1971 to Peter A. Franaszek. In order to decode these codes correctly, the transmission system must have polarity integrity (e.g. for each positive pulse transmitted, a positive pulse must be received). Hence, in a multi-paired cable transmission system, all cable connections (i.e. splices) require tip and ring integrity. Theoretically, such integrity is maintained in all cable transmission systems. However, due to the myriad of cable connections, complete polarity integrity is difficult to realize in a practical system.
It is known that the polarity of a binary signal can be restored utilizing a technique similar to that encountered in radio transmission caused by phase ambiguity. This involves passing the received binary signal through a binary-polarity insensitive circuit which has been precoded prior to transmission by a circuit with the inverse transfer function so that the binary output signal will be identical to that of the input signal.